Where the cause of by-election is given as "resignation" or "seeks re-election", this indicates that the incumbent was appointed on his or her own request to an "office of profit under the Crown", either the Steward of the Chiltern Hundreds or the Steward of the Manor of Northstead. Accepting an office of profit under the Crown vacates the member's seat. This process is used because members of the House of Commons are not technically permitted to resign. A member who vacates their seat in this manner may stand for re-election.

There were 10 by-elections in the 2015–2017 Parliament (with a planned by-election in Manchester Gorton cancelled when the 2017 general election was called). Seven were in seats held by Labour, and three by the governing Conservatives. Eight by-elections were won by the incumbent party, the Conservatives won a seat from Labour and lost one to the Liberal Democrats. Three by-elections were a result of the death of the incumbent MPs, all of whom represented Labour.

At the dissolution of Parliament in 2017 there was one vacancy: Manchester Gorton, caused by the death of its Labour member Gerald Kaufman. With the close proximity of the 2017 general election on 8 June, the by-election previously called for 4 May had its writ cancelled by the House of Commons.[1] All but two of the candidates nominated for the by-election then stood at the General Election, and the seat was held by Labour.

There were 21 by-elections in the 2010–2015 Parliament. 14 were in seats held by Labour, four by the governing Conservatives, one by their coalition partners the Liberal Democrats and two by Sinn Féin, who do not take up their seats in the House of Commons. 17 by-elections were won by the incumbent party, Labour won a seat from the Conservatives and lost one to Respect, while UKIP gained two seats from the Conservatives after the incumbent MPs defected to the party and were re-elected. Six by-elections were a result of the death of the incumbent MPs, all of whom represented Labour.